Flying

How do birds fly? This is a question that humans have tried to answer for thousands of years. From watching birds we know that flapping the wings up and down somehow makes them fly, and yet when man has tried to mimic this flapping motion it has never resulted in flight.

Scientists are only just starting to understand how birds fly; the following is a simplified explanation.

May the Force Be Against You

There are two natural forces that a bird must overcome so that it can fly:

1. Gravity - the force that draws all objects to the ground
   If you let go of an object from your hand, it falls to the ground because of gravity.
2. Drag - the force that slows things down
   If you move your hand, palm facing forwards, through the air, this is the force you can feel on the palm or back of your hand.

The bird must generate a force, called lift, that pushes it away from the ground, and another force called thrust that pushes it forward through the air.

Going Up

Intuitively, the downward flap of a wing beat should create lift, but then why doesn't the upward flap do the opposite? The answer is partly explained by how birds soar and glide.

When a bird is soaring it does not flap its wings and yet it is creating lift so as to remain aloft. This is the amazing, counter-intuitive part: lift is created not by flapping but by air flowing over the surface of the wing.

If we take a slice through a bird's wing its shape is like a teardrop, which is called an aerofoil. When an oncoming stream of air hits the leading edge of an aerofoil it splits into two air streams, one passing over the top of the aerofoil and the other underneath. The air streams below the aerofoil bunch together forming a higher pressure region whilst those above spread apart to form a lower pressure region. The difference in pressure above and below the aerofoil creates lift.

Moving Forwards

Thrust is the force required to overcome drag and drive the bird forwards. The counter-intuitive discovery that lift is created by air flow over the wing prepares us for an equally complicated explanation for how birds create thrust.

If we watch a large bird that has a slow wing beat, such as a member of the crow family, we can see that the wing is not simply flapped up and down. On the downstroke, the wing tip moves downwards and slightly forwards and on the upstroke the tip also moves backwards. This gives the impression that the bird is swimming or rowing through the air. Unfortunately, it is not quite so simple.

Towards the end of the downstroke, the air beneath the wing causes the feathers to twist into a vertical position (see illustration). Each of these flight feathers is also shaped as an aerofoil. With the front facing surface of the feather corresponding to the top surface of the aerofoil (in the illustration above), air passing over the surfaces of the twisted feather creates a forwards pushing thrust.

Flight

There seems to be great extremes in the flight capability of birds, some can:

• Fly at high altitudes, such as migrating Whooper Swans at 8850 metres (29 000 feet).
• Stay aloft for months at a time, such as Swifts and Albatrosses.
• Travel long distances, such as Swifts, and Terns.
• Reach incredible speeds, such as a Peregrine exceeding 300 kph (about 190 miles per hour) while chasing prey.

Generally, however, most birds fly at 15-50 km/h (about 10-30 miles per hour) and at altitudes of less than 150 metres (about 500 feet), to go higher requires more energy and greater exposure to stronger, colder winds and birds of prey. This is a compromise that involves maximising distances and minimising metabolic rates, which in turn determines their energy requirements and how much food they must find and eat.

The shape and size of the wings is chiefly responsible for how a bird flies and not the size of the bird itself. For example:

• Passerines, such as House Sparrows and Blackbirds, have wings that taper to a point (reducing drag) and are short relative to their width which enables rapid take off but not sustained high speed, and agility to escape predators.
• Carrion Crows have "fingers" or slots at the end of the wings to reduce turbulence and so allow them to stay aloft at much lower speeds.
• Swallows and some falcons have long, flat wings that allow high speed flight, but hawks (like Sparrowhawk) and owls that hunt in woodland areas have shorter wings for more manoeuvrability at the expense of less speed.

The skeleton of most birds is greatly modified for flight:

• Some bones are fused together, for example the vertebrae and pelvic girdle.
• Others bones have been eliminated, for example, the fingers.
• Most bones contain hollows to reduce weight, and some are connected to the respiratory system so that they can be filled with air.

Interestingly, some scientists now believe that feathers evolved (see Evolution) primarily for insulation and evolved much later as flight-related features. However, while some feathers are undoubtedly the most efficient natural insulators - just think about eider down in duvets - others are ideally developed for flight.

So which came first, insulation or flight?